For control of plant equipments, there have been typical plant control systems including those configured with control devices interconnected through a network to implement data transfer between control devices via the network. Also, there have been plant control systems for large-scale plants, including those configured with control devices increased in number and accompanied by a set of superordinate networks hierarchically architected to implement data transfer between networks.
Still also, there has been use of a set of sub-network repeaters described in a patent literature 1 (Japanese Patent Application Laying-open Publication No. 5-336118), for instance, to constitute a plant control system.
FIG. 18 is a conceptual diagram showing a concept of data transfer in a plant control system in the past.
As shown in FIG. 18, the plant control system 100 in the past had data transfer systems DT104, DT105, and DT106 interconnected through a network N110. The data transfer system DT104 was connected through a sub-network N101 to n sub-network stations STN111 to STN11n. Likewise the data transfer system DT105 was connected through a sub-network N102 to n sub-network stations STN121 to STN12n, and the data transfer system DT106 was connected through a sub-network N103 to n sub-network stations STN131 to STN13n. 
The n sub-network stations STN111 to STN11n were connected with n control devices PCS111 to PCS11n, respectively. Likewise the n sub-network stations STN121 to STN12n were connected with n control devices PCS121 to PCS12n, respectively, and the n sub-network stations STN131 to STN13n were connected with n control devices PCS131 to PCS13n, respectively.
There was combination of a memory shared to be common in sub-network 101 for STN11Data to STN1nData to be defined, a memory shared to be common in sub-network 102 for STN21Data to STN2nData to be defined, and a memory shared to be common in sub-network 103 for STN31Data to STN3nData to be defined.
In such the configuration, for transfer of data between a certain sub-network and any sub-network else, implemented was a transfer of data through network 110 using data transfer systems DT104, DT105, and/or DT106.
For instance, as illustrated in FIG. 18, for pieces of data STN11Data and STN12Data in the memory shared in the sub-network N101 to be used at a control device PCS121 connected to the sub-network N 102, there was an operation of the data transfer system DT104 to have the pieces of data STN11Data and STN12Data in the shared memory in the sub-network N101 once transferred through the superordinate network N110 to the data transfer system DT105. Then, the data transfer system DT105 was operated, so that the transferred data STN and STN12Data of shared memory were transferred to sub-network stations STN121 to STN12n, as pieces of data to be shared under the sub-network N102.